Photoemissive electron tube comprising a thin film transmissive semiconductor photocathode structure

ABSTRACT

A transmissive semiconductor photocathode structure comprising a first monocrystalline epitaxial layer of silicon or germanium about 200 to 300 nanometers thick on a major surface of a transparent monocrystalline dielectric substrate. On the silicon or germanium layer is a second monocrystalline epitaxial layer of a III-V or II-VI semiconductor compound having a thickness of at least about three microns. On the second layer is a third monocrystalline epitaxial layer of a III-V semiconductor compound having an energy bandgap smaller than the second layer compound and having a thickness on the order of from about one micron to about five microns. Also disclosed is a photoemissive electron tube utilizing the transmissive photocathode structure, with a work function reducing material deposited on the emissive surface of the third layer.

United States Patent 1 Oct. 17,1972

Tietjfn et a1.

[54] PHOTOEMISSIVE ELECTRON TUBE OTHER PUBLICATIONS Marinace, IBM Tech. Discl. Bul1., V01. 6, No. 2 July 1963 721 Inventors: James Joseph Tietjfn, Bell Mead; Primao Examiner-Martin Edlflw Brown F. Williams, Princeton; Chin Attorney-Glenn Bruesue Chun Wang, l-lightstown, all of NJ. t [57] ABSTRACT [73] Assrgnee: RCA Corporation A transmissive semiconductor photocathode structure Flledi May 11, 1971' comprising a first monocrystalline epitaxial layer of [2]] Appl No: 143,862 silicon or germanium about 200 to 300 nanometers thick on a ma or surface of a transparent monocrystalline dielectric substrate. On the silicon or germanium 317/235 317/234 3, layer is a second monocrystalline epitaxial layer of a 1 AC III-V or lI-VI semiconductor compound having a [51] Int. Cl. ..H01I 15/00 thickness of at least about three microns, On the [58] Field of Search ..3 17/235 N, 234 S, secondlayer athirdmonocrystalline epitaxial layer 317/235 AC of a Ill-V semiconductor compound having an energy I Y bandgap smaller than the second layer compound and [56] References C te having a thickness on the order of from about one UNITED STATES PATENTS :licro: to about five microns. I

so isclosed is a photoemissive e ectron tube utiliz- 3,400,015 9/1968 Chapman ..117/22 ing the transmissive photocathode structure, with a KI'CSSCI work f i reducing material deposited on the 3,478,213 I 1/1969 Lemon ..250/207 emissive Surface of the third layer. 3,575,628 11/1968 Word ..313/95 3,433,684 3/1969 Zanowick ..148/33.4 16 Claims, 2 Drawing Figures PHOTOEMISSIVE ELECTRON TUBE COMPRISING A THIN FILM TRANSMISSIVE SEMICONDUCTOR PHOTOCATI-IODE STRUCTURE BACKGROUND OF THE INVENTION The present invention relates to photoemissive electron tubes and particularly to such tubes having a thin film light transmissive mode GaAs photocathode.

Photoemissive electron tubes, such as certain image tubes, camera tubes, and photomultiplier tubes generally contain a photocathode which emits electrons in response to incident light. Gallium arsenide (GaAs) is known to be a highly-efficient photocathode material for visible and near infrared light, particularly if its emissive surface has a negative effect electron affinity. Some gallium arsenide photocathodes are described for instance in the following:

Simon, R. E. and Williams, B. F. Electron Emission From a Cold Cathode GaAs P-N Junction, in Applied Physics Letters, Ap. l, 1969; p. 214-216;

Syms, C. H. A. Gallium Arsenide Thin Film Photocathodes, in Advances in Electronics and Electron Physics; 28A p. 399, 1969.

Reflective photocathodes of GaAs, those for which the light is incident directly on the emissive surface, can be made by sensitizing a clean surface of a GaAs wafer sliced from a high-purity ingot. However, transmissive photocathodes of GaAs, those for which light is incident on the GaAs from the side opposite the emissive surface, require special considerations. For example, GaAs is substantially opaque to most of the light in the visible spectrum. In order for electrons generated in the GaAs by the incident light to reach the vicinity of the emissive surface, the GaAs must be no thicker than the diffusion length of the electrons, which is about one micron. One micron is not enough thickness to provide structural support. The structural support may be provided by growing a thin epitaxial layer of GaAs on a supportingsubstrate of spinel or sapphire as described, for instance, in the second-cited publication above. Superior GaAs layers are obtained by first providing a thin nucleation layer of silicon on the substrate and then growing the GaAs on the silicon to obtain a better match of crystal lattice orientation between the substrate and the GaAs as described, for example, in:

U. S. Pat. No. 3,433,684, issued 18 Mar. 1969 to R.

L. Zanowick et al. (U. 8. Cl. 148-334). However, it is found that thin epitaxial GaAs films grown on a silicon nucleation layer contain atoms of silicon as an impurity and have poor crystallinity at the emissive surface. The silicon impurities and the poor crystallinity degrade the desired long lifetime of minority carriers in the GaAs and thus impede the diffusion of light-generated electrons in the GaAs to the emissive surface.

SUMMARY OF THE INVENTION A semiconductor photocathode structure comprises a transparent monocrystalline dielectric supporting substrate with a first monocrystalline epitaxial layer of silicon or germanium on a major surface. The silicon or germanium layer has a thickness of from about 200 to about 300 nanometers, and its surfaces are in the (100) crystallographic plane. On thefirst layer is a second monocrystalline epitaxial layer of a IlI-V or II-VI semiconductor compound. 0n the second layer is a third monocrystalline epitaxial layer of a Ill-V semiconductor compound having a smaller energy bandgap than the second layer compound.

The invention includes also a photoemissive electron tube utilizing the novel photocathode structure sensitized with a work-function-reducing material deposited on the surface of the layer of the second compound.

The second layer, while being transparent to light which is absorbed by the third layer, provides the thickness of compound needed for obtaining an emissive surface sufficiently free of impurities and having good crystallinity. In addition, the second layer compound can be chosen to provide a favorable optical match between the substrate material and the third layer compounds, so that there is a minimum light loss by interface reflection.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a sectional view of a proximity-focused image tube comprising the novel photocathode structure.

FIG. 2 is a sectional fragment view of the photocathode structure of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS ing flanges 14 and 15 welded to a short glass cylinder 16 to form a double rim. One flange 14 supports an input faceplate 17 and other flange 15 supports an output faceplate 18. The faceplates l7 and 18 are hermetically sealed to the flanges 14, 15 so that the entire envelope assembly can be evacuated through a short piece of exhaust tubulation 20 and sealed. The faceplates l7 and 18 are closely spaced inside the envelope at a distance of about 3 millimeters to minimize defocussing effects. On the inside surface of the output faceplate 18 is a phosphor screen 21 covered with a thin aluminum coating.

The input faceplate 17 is a single crystalline disc of alpha-type aluminum oxide, also known as synthetic sapphire, about one inch in diameter and 25 mils thick. The faces of the faceplate 17 are cut so that they lie in the Miller Index crystallographic plane designated as (1 102) and are optically polished. The output faceplate 18 is optical glass having dimensions on the order of the dimensions of the input faceplate l7.

A photocathode structure 12, about 18 mm in diameter is coated directly on the inside surface of the input faceplate l7 and comprises three epitaxial layers. A magnified portion of the photocathode structure 12 is shown in FIG. 2. There is, beginning from the input faceplate 17, a first layer of silicon 24 about 200 nanometers thick. On the silicon is a second layer, of gallium phosphide 26, about 5 microns thick. On the gallium phosphide layer 26 is a third layer, of gallium arsenide 28, about 1 micron thick, the exposed surface of which is the electron emissive surface 22.

The photocathode structure 12 is grown by epitaxial vapor phase processes directly on the input faceplate 17 as follows: The surface of the input faceplate 17 is mechanically polished to epitaxial grade. It is then vapor rinsed in trichloroethylene for 2 minutes, followed by vapor rinsing in isopropyl alcohol for 2 minutes. Next, the faceplate 17 is mounted in a radiofrequency susceptor block of silicon carbide coated carbon, placed in a horizontal reactor, and heated by radio frequency heating of the susceptor block to about l,200 C in the presence of hydrogen for about 15 minutes to etch the surface. The temperature is then lowered to about 1,050 C and 3 percent silane in high purity palladium diffused hydrogen admitted to the reactor with a carrier flow of 2.25 liters per minute to deposit the silicon layer 24. The thickness of the silicon layer 24 is monitored by light interference fringe methods.

After deposition of the silicon layer 24, the input faceplate 17 is cooled to room temperature, removed from the reactor, placed in a second susceptor block in a vertical reactor. Therein it is heated in the presence of hydrogen to about 900 C for about 10 minutes, after which there is admitted about 5 percent arsine (Asl-l in hydrogen to clean the silicon. Then the input faceplate 17 is then cooled to about 800 C and trimethyl gallium and percent phosphine (P11 in a carrier of hydrogen is admitted to the reactor to grow the gallium phosphide layer 26 for about 45 minutes. The flow rate of the carrier for the trimethyl gallium is 33.8 cc/min. The flow rate for the phosphine carrier is 450 cc/min. The input faceplate 17 is next cooled to room temperature in the presence of hydrogen, removed from the reactor, and etched with a bromine methanol solution containing 3 percent bromine by weight.

After the gallium phosphide layer 26 is deposited, the input faceplate 17 is mounted in a horizontal threezone reactor for depositing the gallium arsenide layer 28 with a heavy zinc doping concentration of about 10 atoms per cubic cm. and a thickness of about 1 micron. The reactor is resistance-heated until the central zone is at about 800 C. Gallium is heated in a second zone of the reactor in the presenceof hydrogen chloride to form a gallium subchloride. Arsine is transported by a high purity palladium-diffused hydrogen carrier from the third zone into the second zone, from where it passes with the gallium sub-chloride to the gallium phosphide layer 26 in the first zone, so that the gallium arsenide layer 28 vapor deposits on the gallium phosphide layer 26. The input faceplate 17 is then cooled in hydrogen.

The faceplate 17 with the now-completed photocathode structure 12 on it is next assembled with the other portions of the tube 10 by indium sealing, and the surface of the gallium arsenide layer 28 sensitized to negative effective electron affinity by treatment with cesium and oxygen to form the photoemissive surface 22. Exposure of the surface of the GaAs layer 28 to air should be avoided, as this generally degrades the final photo-emission. Typical procedures for assembly and sensitizing are described, for instance, in US. Pat. No. 2,975,015, issued 14 March 1961 to D. W. Davis (cl. 3 16-19) and the Syms publication referred to earlier.

In operation of the tube, a light image is focussed through the input faceplate 17 to the photocathode structure 12, which is biased at several thousand volts negative with respect to the phosphor screen 21. In response to the incident light, electrons generated in the GaAs are emitted from the inside emissive surface 22 and travel a short distance to the phosphor screen 21, whereupon striking the phosphor light is emitted through the output faceplate 18.

Example II In a second embodiment of the novel photocathode structure, the input faceplate 17 is of spinel, of generally the same dimensions as the aluminum oxide faceplate of Example I. The surface of the spinel faceplate on which the first epitaxial layer 24 photocathode structure 12 is deposited lies in the Miller Index plane A process for forming the layers 24, 26 and 28 on the spinel are generally the same as for Example I.

GENERAL CONSIDERATIONS The term index plane herein refers to the Miller Index-Plane designations for surface orientations of crystals. It is found that for GaAs photoemissive surfaces sensitized with cesium and oxygen, the photoemission is somewhat higher when the surface of the GaAs is in the index plane (100) than for other possible index planes. Therefore, it is desirable that the substrate surfaces be in the (1 102) plane for sapphire and the (100) plane for spinel. Under these circumstances, the three epitaxial layers necessarily lie in the (100) plane. For the case of the sapphire substrate this is because silicon or germanium epitaxially grown on a (l 102) plane of sapphire has a 100) plane orientation, rather than a 1 102) plane orientation, and this in turn determines the orientation of Ill-V compound or lI-Vl compound layers epitaxially grown on it to be in the 100) plane.

While specific parameters for growth of the Ga? and GaAs layers of Example I are specified, it is to be understood that the values may all be varied to some extent. They are chosen to yield layers of optimum crystallinity for photoemission performance. The silicon or germanium layer need only be thick enough to fully cover the substrate surface, generally about 200-300 nanometers, so that it provides a nucleation layer for the next grown layer.

The second layer compound should have an energy bandgap larger than that of the third layer compound from which the emission occurs, so that this second layer will not absorb light of as long a wavelength as the third layer. The difference in the energy bandgaps may be chosen to suit the wavelengths of light which are to result in photoemission, and to at the same time utilize materials having optimum match of lattice parameters, so that the epitaxial growth of the layers on oneanother results in best possible crystallinity of the emissive third layer.

Although the novel photocathode structure is particularly suited for a GaAs photoemitting layer, the second layer of the structure may be chosen from a number of semiconductor compounds chosen from either groups Illa and Va or groups 11b and Vla of the Periodic Table of the Elements, whereas the third layer may be any of a number of III-V compounds. The compound for either the second or third layer may be binary or ternary. It is essential, however, that the energy bandgap of the second layer compound be greater than that of the third layer compound so that light absorbed by the third layer will pass substantially through the second layer. For instance, when the emissive third layer compound is GaAs, the second layer may be GaP, AlAs, ZnSe, Al? or a gallium-arsenide-phosphide alloy. Gallium-arsenide-phosphide may be graded to provide a better lattice match for the second compound layer. The refractive indices of sapphire, GaP, and GaAs are about 1.7, 2.6 and 2.9, respectively, (the silicon layer is sothin that its index of refraction is unimportant). Thus, GaP provides a beneficial optical matching from the sapphire to the GaAs and thus minimizes loss of incoming light by reflection from an interface.

In order to minimize light absorption in the second layer, the layer should be as thin as is practicable, while being thick enough to provide a surface of good crystallinity for the third layer. A thickness of at least about 3 microns is generally required for the second layer. The maximum permissible thickness is dependent on the light transmissivity of the material. For highly transparent material such as GaP, the layer may be as much as a centimeter thick.

The thickness of the electron-emitting second compound layer is chosen just thick enough so that substantially all the light incident on it is absorbed. Greater thickness results in increased chance of recombination of electrons generated near the light input surface of the material before they reach the opposite emitting surface for emission. The optimum thickness can be readily determined for a particular material by calculation from the observed minority carrier diffusion length for the material and its absorption of light of the wavelength of interest. The optimum thickness for GaAs is at least about one micron for GaAs of such quality that the minority carrier diffusion length is very long as the thickness may be as great as about 5 microns.

We claim:

1. A transmissive semiconductor photocathode structure comprising:

a transparent monocrystalline dielectric supporting substrate having a major surface;

a first monocrystalline epitaxial layer of material chosen from the group consisting of silicon and germanium on said major surface, said first layer having a thickness of from about 200 to about 300 nanometers and having surfaces in the (100) crystallographic plane;

a second monocrystalline epitaxial layer of compound chosen from the group consisting of III-V and ll-Vl semiconductor compounds on said first layer, said second layer having a thickness of at least about 3 microns; and i a third monocrystalline epitaxial layer of a lII-V semiconductor compound on said second layer, said third layer compound having an' energy bandgap smaller than said second layer compound, said third layer having a thickness of from about 1 micron to about 5 microns.

l 0 st 110 ra hic l e. if]? el ectr on emi te r defined in claim 1 wherein said substrate is sapphire having said major surface in the l 102) crystallographic plane.

4. The electron emitter defined in claim 1 wherein said third layer is gallium arsenide.

5. The electron emitter defined in claim 3 wherein said first layer is silicon.

6. The electron emitter defined in claim 5 wherein said second layer is gallium phosphide.

7. The electron emitter defined in claim 5 wherein said second layer is gallium arsenide phosphide.

8. The electron emitter defined in claim 6 wherein said third layer is gallium arsenide.

9. A photoemissive electron tube comprising:

an evacuated envelope having a transparent monocrystalline faceplate with a substantially flat inside surface in the interior of said envelope;

a first monocrystalline epitaxial layer of material chosen from the group consisting of silicon and germanium on said inside surface of said faceplate, said first layer having a thickness of from about 200 to about 300 nanometers;

a second monocrystalline epitaxial layer of a compound chosen from the group consisting of lll-V and Il-Vl semiconductor compounds on said first layer, said second layer having a thickness of at least about 3 microns;

a third monocrystalline epitaxial layer of a III-V semiconductor compound on said second layer, said third layer compound having an energy bandgap smaller than said second layer compound, and said third layer having a thickness of from about 1 micron to about 5 microns, the surface of said third layer being sensitized with at least an electropositive material to reduce the work function to a level below said energy band gap of said third layer compound.

10. The photoemissive electron tube defined in claim 9 wherein said substrate is spinel having said major surface in the crystallographic plane.

11. The photoemissive electron tube defined in claim 9 wherein said substrate is alpha type aluminum oxide having said major surface in the (1102) crystallographic plane.

12. The photoemissive electron tube defined in claim 9 wherein said third layer compound is gallium arsenide.

13. The photoemissive electron tube defined in claim 11 wherein said first layer is silicon.

14. The photoemissive electron tube defined in claim 13 wherein said second layer compound is gallium phosphide.

15. The photoemissive electron tube defined in claim 13 wherein said second layer compound is gallium arsenide phosphide.

16. The photoemissive electron tube defined in claim 14 wherein said third layer compound is gallium arsenide.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION;

'Patent No. 3,699,401 Dated 17 October 1972 Inventor(s) James Joseph Tietjen, Brown F. Williams and Chih Chun Wang It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the inventors names:

"Tiecj fn" should be Tietjen "Chin" should be Chih In the references:

"Lenion" should be Simon Signed and sealed this 24th day of April 1973.-

(SEAL) Attest:

EDWARD M.PLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM po'mso (10'69) uscoMM-oc (scan-ps9 q U.S. GOVERNMENT PRINTING OFFICE: I959 O-.!$6-33 I 

2. The electron emitter defined in claim 1 wherein said substrate is spinel having said major surface in the (100) crystallographic plane.
 3. The electron emitter defined in claim 1 wherein said substrate is sapphire having said major surface in the (1102) crystallographic plane.
 4. The electron emitter defined in claim 1 wherein said third layer is gallium arsenide.
 5. The electron emitter defined in claim 3 wherein said first layer is silicon.
 6. The electron emitter defined in claim 5 wherein said second layer is gallium phosphide.
 7. The electron emitter defined in claim 5 wherein said second layer is gallium arsenide phosphide.
 8. The electron emitter defined in claim 6 wherein said third layer is gallium arsenide.
 9. A photoemissive electron tube comprising: an evacuated envelope having a transparent monocrystalline faceplate with a substantially flat inside surface in the interior of said envelope; a first monocrystalline epitaxial layer of material chosen from the group consisting of silicon and germanium on said inside surface of said faceplate, said first layer having a thickness of from about 200 to about 300 nanometers; a second monocrystalline epitaxial layer of a compound chosen from the group consisting of III-V and II-VI semiconductor compounds on said first layer, said second layer having a thickness of at least about 3 microns; a third monocrystalline epitaxial layer of a III-V semiconductor compound on said second layer, said third layer compound having an energy bandgap smaller than said second layer compound, and said third layer having a thickness of from about 1 micron to about 5 microns, the surface of said third layer being sensitized with at least an electropositive material to reduce the work function to a level below said energy bandgap of said third layer compound.
 10. The photoemissive electron tuBe defined in claim 9 wherein said substrate is spinel having said major surface in the (100) crystallographic plane.
 11. The photoemissive electron tube defined in claim 9 wherein said substrate is alpha type aluminum oxide having said major surface in the (1102) crystallographic plane.
 12. The photoemissive electron tube defined in claim 9 wherein said third layer compound is gallium arsenide.
 13. The photoemissive electron tube defined in claim 11 wherein said first layer is silicon.
 14. The photoemissive electron tube defined in claim 13 wherein said second layer compound is gallium phosphide.
 15. The photoemissive electron tube defined in claim 13 wherein said second layer compound is gallium arsenide phosphide.
 16. The photoemissive electron tube defined in claim 14 wherein said third layer compound is gallium arsenide. 